The present invention relates to a method of blow-molding a biaxially-oriented polyethylene terephthalate resin bottle-shaped container, and the biaxially-oriented polyethylene terephthalate resin bottle-shaped container made according to said process.
Polyethylene terephthalate resin (hereinafter referred to as "PET") has stable physical properties, excellent transparency and high mechanical strength. Also, PET causes no pollution when incinerated. PET is widely used in the production of biaxially-oriented blow-molded bottle-shaped containers, and is particularly useful for bottling foodstuffs.
PET bottle-shaped containers have a number of excellent properties as described above, however, blow-molded bottle-shaped containers of biaxially-oriented PET which are not heat treated suffer remarkable deformation at temperatures of 70.degree. C. or more. Therefore, such PET bottle-shaped containers cannot be used to bottle retort food, which is heat treated by allowing the food to stand for 30 min. at 120.degree. C., or other heat treated food. Accordingly, there is great demand for PET bottle-shaped containers which have high heat resistance.
There are several conventional methods of imparting heat resistance to PET bottle-shaped containers such as (1) heating a blowing mold during blow-molding of a PET bottle-shaped container to a temperature higher than the target heat resistance temperature value to increase the density of the PET bottle-shaped container; (2) heat setting a PET bottle-shaped container after blow-molding to remove residual strain produced by blow-molding; and (3) blow-molding an intermediate molded piece by first molding a primary blow-molded container, then reheating it at approximately 110.degree. C., and finally blowing it again to produce a bottle-shaped container.
In method (1), the moldability of the PET decreases as the mold temperature rises. According to this method, the PET is heat resistant up to a maximum of approximately 100.degree. C. This PET cannot be used for bottle-shaped containers containing food which is heat treated at temperatures much higher than 100.degree. C. Methods (2) and (3) of imparting heat resistance to a PET bottle-shaped container cannot expect to produce a heat resistance higher than that of method (1).